SvLEN is the size of the buffer currently referenced by SvPVX.

I didn't say a scalar's buffer couldn't be replaced with a smaller one (which would result in a reduction of a scalar's SvLEN). But even so, it doesn't replace a scalar's buffer unless necessary either.

That's why copying a short string over a long one in a buffer doesn't affect the buffer size (in any version of Perl). 

use Devel::Peek qw( Dump );
$_ = "x" x 999;
$_ .= "x";  # Force unsharing.
Dump( $_ );
$_ = "abc";
Dump( $_ );

[download]
SV = PV(0x5b5d1f3fdee0) at 0x5b5d1f439bc8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x5b5d1f441580 "xxx[...]xxx"\0
  CUR = 1000
  LEN = 1001
SV = PV(0x5b5d1f3fdee0) at 0x5b5d1f439bc8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x5b5d1f441580 "abc"\0    <-- Same buffer
  CUR = 3
  LEN = 1001                     <-- Same size

[download]

And that's why the difference between when SvLEN goes down or not is whether Perl must allocate a fresh buffer or not.

OOK is a mechanism which can be used used to efficiently delete from the start of a string. Rather than shifting the entire contents of the buffer, OOK can be used to fake the start and size of the buffer instead. 

use Devel::Peek qw( Dump );
$_ = "abcdefghi";
$_ .= "j";  # Force unsharing.
Dump( $_ );
substr( $_ , 0 , 1 ) = "";
Dump( $_ );

[download]
SV = PV(0x600760853ee0) at 0x6007608a35d8
  REFCNT = 1
  FLAGS = (POK,pPOK)
  PV = 0x60076086a2e0 "abcdefghij"\0
  CUR = 10
  LEN = 16
SV = PV(0x600760853ee0) at 0x6007608a35d8
  REFCNT = 1
  FLAGS = (POK,OOK,pPOK)
  OFFSET = 1
  PV = 0x60076086a2e1 ( "\x01" . ) "bcdefghij"\0
  CUR = 9
  LEN = 15

[download]

There's still a 16 byte buffer at 0x60076086a2e0, and the scalar's buffer was replaced with a a 15 byte virtual buffer at 0x60076086a2e1.

SvCUR is offered as a contrast. SvLENis the size of the buffer, and SvCUR is the portion used. Also, someone looking for SvCUR might have landed on SvLEN.

In reply to Re^5: [XS] sv_setpv change in behaviour with perl-5.42.0 and later by ikegami
in thread [XS] sv_setpv change in behaviour with perl-5.42.0 and later by syphilis

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